This is a continuation of a double blind placebo controlled multi-center clinical trial to determine if estrogen can delay the onset of Alzheimer's Disease (AD) and reduce memory decline. The use of estrogen to prevent or delay AD is supported by biologic, epidemiologic and clinical studies. New data are available which demonstrate that estrogen does not have a role in prevention of heart disease and stroke. However, the mechanisms through which estrogens may have a benefit in dementia are not the same as those which may mediate vascular activity. In the original proposal we postulated that estrogen had an effect on cognition and dementia, that the trial to assess dementia in a cohort with high risk of dementia was feasible and that dementia prevention was a high priority. It was designed to answer the question in a population at risk for dementia. We have carefully selected women at high risk for dementia and by careful screening we have selected to reduce the risk of the known estrogen related adverse outcomes. We propose to continue this 5 year multicenter, randomized, double blind placebo controlled trial of estrogens (Premarin or Prempro) to assess its efficacy to prevent memory loss and dementia in 900 healthy, elderly women greater than or equal to age 65 with a family history of AD recruited at 27 sites. Subjects will be assessed at 6 month intervals for safety and compliance and at annual intervals for cognitive outcomes. We believe that this trial must be completed to answer this important question. We propose the following specific aims: 1) to continue the double-blind placebo controlled 5 year trial of Premarin (.625 mg/day) or Prempro to assess the efficacy to prevent memory loss and dementia in healthy elderly women with a family history of AD; 2) To assess the safety of this regimen in this cohort with specific attention to the recently established profile of vascular adverse events; 3) To assess our use of a very sensitive neuropsychotogical battery which may permit reliable, early detection of impaired cognitive health. In an aging society, with the increasing risk of Alzheimer's disease and memory loss, and in a world of growing technological complexity requiring intact cognition, it would be short-sighted to abandon studies of an agent which has the potential to prevent cognitive loss and AD. This trial may represent the very last chance to determine if estrogen can have a benefit in dementia prevention and memory protection.